log: resolve --follow thoroughly in getlogrevs()
This makes sense because --follow isn't really an option to filter revisions,
but an option to extend revisions to be filtered.
_fileancestors() is a minimal copy of revset._follow(). They are slightly
different in that which revision the matcher sees. _fileancestors() also
uses ctx.walk() instead of ctx.manifest().walk() to show a better warning
on bad match, which will be tested later.
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1